The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During wafer fabrication, wafers undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of integrated circuits, an extremely flat surface is needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. As the size of integrated circuits continues to decrease and the number of microstructures per integrated circuits continues to increase, the need for precise and extremely flat wafer surfaces is growing in importance. Accordingly, it is usually necessary to polish or planarize the surface of the wafer between each processing step in order to obtain the flattest surface possible.
Chemical mechanical planarization ("CMP") machines are often utilized for polishing and planarizing semiconductor wafers. Such machines are well known in the art and are described in, for example, Arai, et al., U.S. Pat. No. 4,805,348, issued February, 1989; Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud et aL, U.S. Pat. No. 5,329,732, issued July, 1994; Karlsrud, U.S. Pat. No. 5,498,196, issued March, 1996; and Karlsrud et al., U.S. Pat. No. 5,498,199, issued March, 1996. These references are incorporated herein by reference.
CMP methods for polishing wafers generally involve attaching one side of a wafer to a flat surface of a wafer carrier or chuck, and pressing the opposite side of the wafer against an abrasive top surface of a polishing pad. The abrasive top surface of the pad incorporates an abrasive material such as cerium oxide, aluminum oxide, fumed/precipitated silica or another particulate abrasive, while the underlying pad is formed from a commercially available material such as blown polyurethane. A commercially available pad, such as the IC 1000, SUBA IV or GS series pads from Rodel Products Corporation of Scottsdale, Ariz., may be utilized. The hardness and density of the polishing pad is typically dependent upon the material to be polished.
During polishing, the workpiece (e.g., wafer) is pressed against the polishing pad surface while the pad rotates about its vertical axis. The wafer may also be rotated about its vertical axis and radially oscillated back and forth over the surface of the pad to augment the polishing process. Because polishing pads tend to wear unevenly, a conditioning device is often utilized to remove surface irregularities from the pad and to ensure accurate planarization and polishing of all workpieces. The conditioning device may take the form of a conditioning ring separately mounted on an operating arm which contacts the pad remotely from the wafer carrier (ex situ conditioning), or it may be a conditioning ring surrounding the wafer carrier which conditions the pad during wafer processing (in situ conditioning). In situ conditioning is described in detail in U.S. patent application Ser. No. 08/683,571, filed Jul. 15, 1996 and entitled "Method and Apparatus For Conditioning Polishing Pads Using Brazed Diamond Technology, which is incorporated herein by reference.
Although known conditioning devices typically remove most localized pad surface irregularities, pad wear still causes fluctuations and unpredictability in the overall pad flatness or profile. As illustrated in FIG. 1, pad flatness 156 is generally measured by calculating the difference between the pad's outer diameter thickness or profile 146 and the pad's inner diameter thickness or profile 144. When pad outer diameter thickness 146 is less than pad inner diameter thickness 144, the pad assumes a convex flatness profile. Conversely, when outer diameter thickness 146 is greater than inner diameter thickness 144, the pad assumes a concave flatness profile.
For optimum polishing effectiveness, the pad flatness profile should mirror the shape of the wafer surface being polished. A wafer having a convex flatness profile, for example, should be polished by a pad having a mating concave profile. Hence, rather than having a completely flat profile, the pad must usually possess some degree of concavity or convexity. Uneven polishing can still result when the polishing pad assumes too much of a convex or concave profile. Over polishing in the peripheral regions of the wafer, often referred to as "edge-fast" polishing, may occur when the profile of the pad is overly concave. Conversely, over polishing of the center region of the wafer, referred to as "center-fast" polishing, may occur when the pad profile is overly convex. To minimize such uneven polishing, continuous monitoring and maintenance of a proper pad flatness profile is necessary.
One known method for monitoring pad flatness is to measure material removal rates from different portions of the wafers after polishing. These measurements indicate whether edge-fast or center-fast polishing has occurred and, in turn, indicate whether the pad became overly concave or overly convex during polishing. This method, however, is not practical for large-scale wafer processing operations.
Another known method for monitoring pad flatness is through the use of mechanical flatness gauges, such as "Accu-flat" gauges manufactured by SpeedFam Corporation of Des Plaines, Ill. (see also Cesna, U.S. Pat. No. 4,693,012). Such gauges measure pad thickness at the pad's inner and outer diameters. The difference between the inner and outer diameter measurements indicates the convex or concave nature of the pad's profile.
A typical flatness gauge 400 is illustrated in FIG. 2. To measure inner diameter thickness, the base of gauge 400 is placed on top of wet polishing pad 126 while its indicator tip 127 is placed in contact with the exposed metal surface of polishing wheel 128 underneath the pad. To measure outer diameter thickness, a wedge 402 must usually be cut in the outer edge of pad 126 to allow indicator tip 127 to contact the metal surface below.
Placing gauge 400 on top of pad 126 in this manner poses a number of problems. For softer pads, such as the commercially available IC-1000 pads, the base of the gauge tends to sink into the pad, preventing accurate measurement of the profile. Moreover, the gauge may introduce contaminants onto the pad or scratch the pad's surface. Finally, flatness gauges typically cannot be used for in situ monitoring and maintenance of pad flatness, and are too time consuming for use during ex situ conditioning.